The present invention is directed to a light transmission device for transmitting light between a stationary and movable location. The light transmission device is employed, for example, in electronic reproduction technology.
A light transmission device for transmitting light between a stationary and a movable location is usually composed of at least one fiber optic light waveguide and a movable guide element for the fiber optic light waveguide, for example, cable chains are employed as guide elements in traditional light transmission devices. The fiber optical light waveguide is composed of an actual light conductor which can be an individual fiber or a bundle of fibers and of a hose-like cladding or protective sheath.
Light transmission devices of this type are employed, for example, in electronic reproduction technology in a reproduction device for producing half-tone color separations or rastered color separations by an opto-electronic scanning of an original and by exposing a recording medium. In an electronic reproduction device for producing half-tone color separations, a bundle or beam of coherent light is generated in a laser light source. This beam of laser light is modulated in brightness in a stationary modulator by an image signal which was acquired in the original scanning. The modulated beam of laser light is transmitted to a recording element from the stationary modulator via a light transmission device comprising an individual fiber. The recording element moves axially along the recording drum on which the recording medium is clamped. The modulated beam of laser light emerging from the exit face of the individual fiber in the recording element is imaged on the recording medium as an exposure point by an arrangement including a diaphragm and a lens. This recording medium is then exposed point-by-point and line-by-line due to the relative motion between the recording element and the recording drum.
In electronic reproduction devices for producing rastered color separations, a plurality of coherent divided light beams or bundles are generated in a laser light source and these divided light beams are separately switched on and off in a stationary modulator by the image signals and are then transmitted via individual fibers of a fiber bundle of the light transmission device onto the moving recording element. In the recording element, the exit faces of the individual fibers are juxtaposed and are situated next to one another on a generated line of the recording drum so that the modulated divided light beams emerging from the individual fibers are imaged on the recording medium by means of a lens as a plurality of juxtaposed exposure points lying side-by-side. Due to the relative motion between the recording drum and the recording element, the exposure points generate write lines lying side-by-side whose lengths are respectively dependent on the on-time of the individual divided light beams. Every raster point is composed of such write lines so that the shape and size of the raster point depends on the length of the write lines or, respectively, on the respective on-time of the divided light beams.
Since, due to the small core diameter of a monomode-type light waveguide, it is very difficult to couple coherent light in a stable fashion into the monomode light waveguide, light waveguides of a multimode-type are usually employed for the light transmission. When, however, coherent light is coupled into the light waveguide of the multimode-type, then a plurality of modes are formed which, due to the mode interference, produce a mode pattern at the exit face thereof in the form of bright and dark spots and thus produce an inhomogeneous light distribution in the exit face.
An additional complicating factor is that the mode pattern shifts in the exit face due to bending and/or twisting movement of the light guide so that the light distribution and, in addition, the light intensity will chronologically change at the exit face. The problem of mode formations are disclosed, for example, in the article by R. E. Epworth, "Modal Noise-Causes and Cures", Laser Focus, September 1981, pages 109-115 and from an article in "Lichtleiter als Sensoran" Elektronik, No. 19, 1981, page 16.
With the employment of a traditional light transmission device, which comprises a cable chain and a light waveguide of a multimode-type extending between a stationary modulator and a displaceable recording element, changing friction conditions of the fiber optical light waveguide necessary occur when the cable chain uncoils. Thus, a continuous and discontinuous change of the mechanical tension between the core and jacket of the light waveguide on the one hand and between the light waveguide and the protective sheath on the other hand will occur from this changing frictional condition.
In an electronic reproduction device for producing half-tone color separations, the continuous or discontinuous change of the light distribution in the exit face of the light waveguide, which changes are caused by the motion sequence, leads to an inhomogeneous and chronological changing illumination of the diaphragm or, respectively, light distribution within the exposure spot which is further intensified due to the aperture limitation of the diaphragm. This causes density changes on the recording medium so that a half-tone area which is homogeneous per se is exposed in a streaky fashion. In a reproduction device for the production of rastered color separations by contrast, the chronological intensity changes differing in the individual divided light beams particularly lead to different densities in the write lines or, respectively, raster points. In both cases, disturbing density changes on the recording medium, which considerably deteriorate the reproduction quality, occur in the color separation.
It has been shown in practice that a gradual, large-area density change caused by a uniform motion or, respectively, tension changes of the light waveguide is not as noticeable to the human eye in a distrubing way as discontinuous, small-area density changes caused by a discontinuous motion or, respectively, tension change.
In order to avoid a streaky exposure in an electronic reproduction device for half-tone color separations, a process and arrangement is disclosed in U.S. Pat. No. 4,439,022, whose disclosure is incorporated by reference thereto. As disclosed by this process and apparatus, the modes emerging from the exit face of the light waveguide in the recording element are fanned with a scattering medium, the fanned modes are then superimposed in a superimposition range with a positive lens and the diaphragm is situated in the superimposition region so that a homogeneous diaphragm illumination and a uniform intensity distribution are always achieved for the exposure point. The disadvantage of this known optical arrangement is that it can only be employed when recording half-tone color separations.
Another solution for avoiding a streaky exposure is disclosed in U.S. Pat. No. 4,523,805 whose disclosure is incorporated by reference thereto. This solution is based on the perception that when the light waveguide is bent, the mode spots predominantly migrate into and out of the exit face, which is assumed to be circular, in a radial direction and thus migrate into and out of the diaphragm aperture so that a great chronological change of the light distribution will occur within the diaphragm aperture. In contrast thereto, a mode spot migrates in a circumferential direction of the circular exit face given twisting of the light waveguide and, thus, within the diaphragm aperture so that the chronological change in the intensity distribution is diminished. The guide element of the light transmission device is fashioned a pivotable lever mechanism. This lever mechanism supports the fiber optical light waveguide so that it is only twisted and not bent. This solution, however, is extremely involved.